Production of metal borides by fused salt electrolysis



April 10, 1956 s. J. SINDEBAND 2,741,587

PRODUCTION OF' METAL BORDES BY F'USED SALT ELECTROLYSIS Filed June 24. 1950 ff 5 f7 a ,6

IN V EN TOR 5, J. S//vof//Na BY PRODUCTION F 'METAL BORIDES BY FUSED SALT ELECTROLYSIS Seymour J. Sindeband, Chappaqua, N. Y., assigner, by mesne assignments, to Eurolite Corporation, Pittsburgh, Pa., a corporation of Delaware Application June r24, 1959, Serial N 170,239

4 Claims. (Cl. 204-61) This invention relates to the production of metal borides by fused salt electrolysis, and equipment for effectively carrying on such electrolysis.

ln the past critical difculties have been encountered in producing metal borides by fused salt electrolysis in the manner suggested in the literature. When producing metal borides by fused salt electrolysis, the bath must contain a sufficient amount of the metal compound, such as the metal oxide, in order to form on a practically etilcient basis, the desired metal boride. However, this has heretofore been practically impossible because various obscure reactions occurring Within the vfused salt bath during the progress of the electrolysis results in the production of defective borides, limit the yield, and render the process inecient.

According to one phase of the invention, the critical diiiculties resulting in the production of defective borides are overcome by carrying on the fused salt electrolysis in such manner as to suppress and substantially prevent the presence of an excess of .the metal compounde-such as the metal oxide constituent of the bath-n the form of finely dispersed particles While maintaining at all times dissolved within tue fused salt bath the proper concentration of the metal compound required for reiiiciently carrying on the boride forming electrolysis process.

ln accordance with theinvention, the metal compound, such as the metal oxide, is maintained Ywithin the bath at the proper concentration required for ecientvproduction of the borides, while preventing the presence of iely dispersed oxide particles within the `bath by placing in the bath the metal oxide in the form of relatively large solid pieces or structures, Vso vthat it may dissolve in the fused salt bath as it is being used up as the electrolysis proceeds.

According to another phase of the invention, the eiciency with which the desired ,metal boridestare produced, is greatly increased by providing out of themetal'compound-such as the metal oxide which must be present as an essential ingredient of the fused salt bath-aporous barrier separating the anode region of 'thebath from the cathode region within `which the desired metal boride formed by electrolysisis deposited.

ln accordance with the invention, a porous solid body or bodies of the metal compound, such' as metal oxide, are placed therein in the form Vof a beaker-like structure, or in the form of porous solid 'lumps or blocks-arrayed or otherwise positioned Ywithin the bath so as to provide a porous barrier separating 'the anode region from the cathode region ofthe bath, which usually has the form of an electrically conducting vessel of `refractory material such as carbon, vholdingtherein the Afused molten salt bath.

The forecoing and other objec of the invention will be best understood from the 'following description of exemplications of the invention, reference being vhad Vto the accompanying drawings wherein:

Fig. 1 is a diagrammatic top view of one y'form of itecl States Patent i ce ,en, Z

equipment for producing metal borides by fused salt electrolysis in accordance with the invention; and

Fig. 2 is a vertical cross-section of the equipment of Fig. l.

The present invention is applicable to the production by fused salt electrolysis of various metal borides including the borides of titanium, tantalum, columbium, zirconium, hafnium, vanadium and chromium. By way of example, there will now be described the process of producing zirconium boride by lfused `salt electrolysis in ac cordance with the invention, a generally similar process being suitable and highly effective for producing the other metal borides.

A fused salt bath which has been found very effective in practice for producing refractory metal borides in accordance with the invention is one containing calcium oxide CaO, calcium fluoride CaF2, the oxide of Vthe desired metal, and boron oxide B203. Instead or" calcium oxide, other alkali metal or alkaline earth metal oxides or mixtures thereof may be used. Instead of calcium fluoride, the other alkali or alkaline earth halides, such as alkali or alkaline earth iiuorides lor chlorides, or mixtures thereof, may be used. Instead of the metal oxide, other metal compounds which will be held dissolved in the fused salt bath may be used, such as potassium zirconium fluoride instead of zirconium oxide. instead of boric anhydride B203, a borate may be used, making the addition of calcium oxide totally, or partly unnecessary.

When producing zirconium boride or other refractory borides by fused salt electrolysis, diliculties are encountered due to disturbing side reactions which result in the production of defective borides, limit the yield, and lower the overall efliciency, unless the'process is carried on under certain critical conditions.

The formation of the desired metal borides by electrolysis in the fused salt bath consumes the metal oxide ingredients of the bath. In order to maintain the eiciency of the electrolysis process, provision must be `made for continuous replacement of the metal oxides consumed in formation of the metal borides. However, it was found that if any excess undissolved metal oxide is present the form of finely dispersed particles within lthe fused salt bath, the oxide particles serve as ynuclei for the deposition or the Vmetal boride. Metal -boride particles formed around oxide nuclei are of inferior quality and will not yield cemented metal boride bodies of the desired physical characteristics.

According to the invention, the fused salt bath is maintained substantially free of ne metal oxide particles and the required excess of metal oxide for the bath is provided as follows: The metal oxide concentration of the fused salt bath is kept down 'to amounts which are readily dissolved in the fused salt'bath, and the required excess is supplied by placing in the bath solid oxide bodies, such as lumps or pieces which cannot be kept in suspension within the fused salt bath, and cannot serve as nuclei for the formation of metal boride within the bath.

This arrangement `avoids vthe critical difficulties encountered with prior art processes. In addition,'it greatly increases -the eiiiciency of the electrolysis process, and results in the formation of metalboride particles of greater purity, and which are substantially free from vparticles having metal oxide nuclei. ln practice the electrolysis is Acarried on as long as the rate of yield is maintained at .the desired rate, and is stopped after the rate .of yield starts decreasing.

According to another phase of Ythe invention disclosed herein, other critical difficulties are overcome by keeping the vanodetand cathode products fproduced by the electrolysis separated from eachother within the fused salt bath. Tofthis end, the metaloxide ofFthe desiredboride, such as zirconium oxide, is prepared in .the form y,of a

porous crucible structure or enclosure surrounding the anode region within the fused salt bath and keeping it separated from the cathode region wherein the metal boride produced by electrolysis accumulates. The barrier enclosure may be conveniently formed by piling up shaped blocks'resernbling building blocks, to form a porous barrier extending across the fused salt bath, and

separating the anode region from the cathode region, or I regions thereof.

Vhaving positive and negative terminals indicated by sign i4, and (-l-) sign l'respectively. The negative direct current terminal 14 is shown connected to the graphite crucible it?, serving asvthe cathode. The three graphite anodes 13 are connected through a common lead Y 16 to Vthe ,positive direct current terminal l5.

ln operation, the fused salt bath 11 is heated by passing an alternating electric current between an additional electrode 17 connected to the terminal i8 of a source of heating current and one of the other electrodes, such as the anode electrodes i3 which are shown connected to a lead l9'to the other terminal 13 of the heating cur-V rent, lead T19 forming arbrancn of the anode lead 16 connected to the positive direct current terminal 1'. The heating circuit electrode i7 mayV likewise be a graphite electrode, and an alternating current source is shown used as a source of heating current connected to the two e heating current terminals 13 which are labelled by the legend (ac). Y By way of example, the following fused Vsalt bath composition was found to give a good yield when used for producing zirconium boride:

integral hollow structure of the metal oxide, such as zirconium oxide, which has suicient porosity as to permit the electrolysis process to proceed between the anodes 13 and the cathode 10. With a barrier arrangement of the invention of the type described above, the efficiency Y and yield in the production of the desired borides was increased 50%. The vessel-like barrier structure of zirconium oxide or the other desired metal oxides may be formed ont of the metaloxide particles by the conventional, known ceramic processes used in making vessels or beakers of aluminum oxide or other refractory metal oxides. Y

As indicated in the drawing, the shield or barrier structure 21 is made of segmental porousV blocks, piled and assembled into a hollow porous barrier' vessel enclosure surrounding the region of the bath within Vwhich the anodes 13 are positioned, and separating it from the cathode region of the bath,rextendi ng along the inner sur- Y face of the Crucible 10 serving as a cathode.V

rl`he metal oxide blocks serving as the barrier vessel 2l may be constructed in Va manner analogous to toy build ing blocks with interlocking male and female portions so that when assembled they will remain assembled as a porous barrier 21.

In initially starting the operation of such fused salt electrolysis bath, the properly proportioned ingredients are mixed in powder form and placed within the graphite vessel 10 serving as the cathode. The barrier wall vessel 21 of the metal oxide blocksY is positioned within the vessel before placing therein the powdered bath ingredients. Alternatively, a layer of the metal oxide blocks is iirst deposited on the bottom wall of the vessel 1) before placing thereover the metal oxide barrier wall 21,

whereupon the `vessel 10 is filled with the mixedV Y the mixed bath ingredients in the manner described VGood results are obtained with the several ingredients Y Y supplied, either in the technically pure grade or in the commercially pure grade. in originally preparing the bath, Ythe properly proportioned ingredients are kept Y ready mixed in powder form.V The radditional supply of the metal oxide which is used up as the electrolysis proceeds, is provided in the form of lumps or pieces positioned within the bath, so that they may dissolve therein Vand keep the bath at the desired concentration.

In accordance with a phase of the invention disclosedV herein, the additional lumps of the metal oxide, are positioned in the bath so thatY they form a porous barrier structure 21 enclosing the sides and the bottom region of the anodes k13 and separating the bath region adjacent Y the cathode 10 to'prevent reactions between anode products and the metal borides which are formed by the electrolysis and accumulate at the cathode. ln an equipment having a plurality of individual anodes 13 immersed in the bath, each of the anodes 13 may be sur'- rounded withV its individual porous barrier enclosure separating the region adjacent each anode from the other portions of the bath extending toward the cathode 10. Good results are obtained by providing a single porous continuous vessel-like barrier enclosure 21 separating the region within which the array of anode electrodes 13 is positioned from the cathode region of the bath. The

barrier structure'zl maybe made in the form of a porous above, a molten pool of bath ingredients is produced 1n the region between the heating electrodes 13 and 17 of opposite polarity as by heating it with a torch applied thereto. Once the molten pool of bathV ingredients is formed, further heat energy is supplied by heating current owing between the electrodes i3 and 17 from the heating terminals 18 of opposite polarityY which may be supplied, for instance, through the secondary side of an alternating current supply transformer. After the vcontents of the entire vessel iti-except the metal oxide lumps of the porous barrier Zit-have been molten, the fused salt bath is ready for starting the electrolysis process by closing the direct current circuit. The amount of heating current supplied during the electrolysis is adjusted so as to maintain the bath temperature at a desired level, which has been found to be between l000 C. and l200 C. As the level of the molten bath drops, additional .bath ingredients may be added from the previously prepared properly proportioned mixture of powder ingredients, the lumps or blocks of the metal oxide maintaining the desired metal oxide concentration within the bath. Y

It is also possible to prepare rst a salt mixture which does not contain the Voxide of the boride-forming metal: This mixture is introduced into the furnace, heated with a torch until a molten pool is formed and then kept in the liquid state by the heating current. At this stage, the oxide of the boride-forming metal is introduced by adding a salt mixture containing a proportion of this oxide which is large enough to attain the propor-y tion desired in the final bath. By way of example, in the production of zirconium Vboride, two mixtures, A

faam-,5er

l and B are prepared. The compositions correspond to the following proportions:

M xtl fre A About 3 parts (by weight) of Mixture A are introduced into the furnace. After this mixture is melted, by applying first the torch and then the heating current, l part of Mixture B is added to the bath.

The formation of boride consumes the oxide of `this metal. Eficient electrolysis requires, continuous replacement of the oxide consumed. According to the invention, the desired excess of the metal oxide is provided in solid form and placed in the bath so that it will be dissolved by the bath at a rate corresponding to the oxide consumption. Excess undissolved oxide in the form of finely dispersed `particles was found to serve as nuclei for the deposition of boride. Boride depositions on oxide nuclei must be avoided, since boride particles thus formed contain -oxide cores and are of inferior quality. This diilculty was overcome by keep,- ing the oxide concentration ofthe voriginal. composition down to ramounts which are readily dissolved, and by providing the required excess of solid oxide in Athe form of lumps or piecesV which are not kept in Vsuspension j and cannot serve as nuclei. It was also found that cathode and anode regions of the bath must be separated by porous barriers in order to prevent interaction of cathode and anode products which interaction manifests itself in continuous decrease'of overall current eiciency with increased time of the run. Since the oxides of the boride-forming metals are available in porous form, the vseparators can be fabricated of these materials. Thus, beaker-shaped porous barriers enveloping the anodes of the bath Vcan take the place of the added lumps and provide the supply of -metal oxide 'required for replacement of dissolved oxide consumed by the electrolytic reaction.

By arranging 'the Ametal loxide lpieces vimmersed in the molten bath in 'the form of fa barrier separating the cathode region of the bath from the anode region, disturbing side reactions between anode products and cathode products are suppressed. In particular, such barrier prevents metal vborides which accumulate at the cathode from being carried to 'the Vanode region, and their decomposition.

in carrying on the electrolysis, it is good practice to have a ready prepared mixture -fofthe bath ingredients so that, as the level of the bath drops, it is replenished by adding thereto suitable quantities of the available mixture of its ingredients.

The desired metal borides may be produced at a satisfactory eiciency with a procedure and equipment ot the type described above by electrolysis carried on with a cathode current density in the range between about 7 to l2 amp/sq. in. (amperes per square inch). in practice, the process has been carried on effectively for a long period of time with a cathode current density of about l0 amp/sq. in.

The metal borides which are produced adjacent the surface of the cathode vessel 10 accumulate at its bottom, and they may be withdrawn therefrom through a suitable gate passage lined with carbon. in order to protect the molten metal bolides from freezing into a solid block of great hardness and difcult to break, they are discharged into circulating water so as to '6 quench and breakup tizio-discharged boride body into a conglomenateof frozen particles and lumps.

To remove the contents cr a furnace vessel l0, which is not provided with a gate, the current is .shut off, and its entire contents are poured into circulating water, thereby quenching and breaking them up into -a conglomerate of frozen lumps and particles.

Other metal horides, including borides of titanium, tantalum, columbium, chromium, vanadium and hafnium, may be formed in a similar way by electrolysis in a fused salt hath carried on in accordance with the principles of the invention for producing the desired high purity borides.

In order to separate the metal .boride particles from the impurities, the boride agglomerato with its impurities, is subjected to comminution treatments whereby it is broken down and comminuted into particies having an average particle size of about l00 mesh, thereby loosening the metal boride particles from the metal oxide particles, and other substances, and making Vit possible to subject the mass of powder particles to an initial separation treatment.

To this end, particles 'and lumps of the quenched agglomerate Vare dried and crushed toa uniform particle size, such as 2O mesh, which permits their further cornrninution to mesh by ball-miliing. The com 'minuted mass of loose particles having an 'averagerparticle size .of about -i00 mesh is then subjected to `a `gravity separation treatment, as by a shakin'y table vwhich separates the particles having the same density as the boride particles from particles of lower density.

The resulting mass of loose particles, having'about the same density as the 'refractory boride particles, 'is then leached with hydrochloric acid solution in Aconcentration of 111.5., and -then -washed with waterto remove all Atraces lof acid, yielding a mass of particles containing zirconium boride and zirconium oxide, the other impurities having been dissolved bythe hydrochloric acid.

The tailings from the shaking table may contain boron oxide, calcium oxide, 'calcium uoride, zirconium oxide, `and 'also about 1% or less `of zirconium boride. The tai1- ings may be vre-used for preparing a mixture'for thefused salt bath with Vproper 'adjustment 'for the composition of the tailings. The middlings Afrom the shakingl table Yare re-'passed thereover toobtain afurther separationbetween theyield of particles having'thefsame density as the metal boride andrailings of lower density.

The puriiied :and clean .loose powder mass of an average particle size of -100 mesh and about the-same density as Vthe metal `boride particles, and containing substantially Vonly metal 'boride particles and metal `oxide particles, is then subjected to a dotation treatment which separates the metal boride particles foim the metal oxide particles, yielding, after washing and drying, 'a body of loose .refractory metal boride particles about SS 'to 99% pure` The zirconium boride powder of'98% 'to 99% purity is then ball-milled to size under a cover of .purified mineral oilin an atmosphere of purified argon to reduce the powder particles to an average size of about 2 microns, or Iin general of the order of l to 3 microns. If the ball-milling to size is carried on in a steel ball mill, the ball-milled powder particles are subjected to a leaching treatment with very dilute sulphuric acid in a concentration of l to 30 for dissolving the iron contents, whereupon the powder is washed with water, followed by washing with alcohol, and drying.

By way of example, the following compositions of a fused salt bath are suitable for producing zirconium boride by electrolysis in accordance with the principles of the invention:

In general, the same procedure is followed in producing by electrolysis in a fused salt bath other refractory metal boride powders, including theborides of titanium, tan- Y talum, hafnium, chromium, columbium and vanadium, in

a form suitable for use in refractory cemented metal boride compositions, by substitting the oxide of the desired other The titanium oxide of the bath may be replaced by potassium titanium uoride KzTiFs.

Forv the production of tantalum boride, a fused salt Y' bath of the following composition will give satisfactory results; Y

` Ta2O5-i-20B2Os-I- l0CaO -l- 10CaF2 For the production of columbium boride, a fused salt bath Vof the following composition will give satisfactory results: Y

For the production of chromium boride, a fused salt bath of the following composition is satisfactory:

xcolumbium, chromium, vanadium and hafnium by electrolysis within a fused salt'fbath, the procedure of providing a fused salt bath consisting of a metal compound of the metal of the desired boride and of additional bath ingredients within which said metal compound will dissolve at Yelevated temperatures in the range between about 900 c to 1300o C. at which the ingredients of said bath remain molten, with said additional bath ingredients comprising a boron oxide and a substance selected from the group consisting ofY alkali metal oxides, alkaline earth metal oxides, alkali metal salts and alkaline earth metalsalts and mixtures'thereof; maintaining within said bath a substantially continuous vessel-like porous barrier consisting predominantly of said metal compound, heating the contents of said bath to an elevated temperature within said range, at which said contents remain in molten condition; passing direct current between an anode immersed in one region vof said bath separated by said barrier from a remaining region of said bath and a cathode in contact with said remaining region of saidrbath for effecting therein an electrolysis and forming said metal boride deposited in the region of said cathode whereby, during the electrolysis, the solvent action of said molten bath on said porous barrier automatically maintains the effective Vconcentration of the metal compound Within the molten bath down to an amount suficientlylow as to maintain said metal compound substantially fully dissolved in the molten 8 bath and the formation of dispersed undissolvedparticles of said metal compound within said molten bath'is substantially suppressed. Y

2. In producing metal boride of a metal selected from the group consisting of zirconium, titanium, tantalum, columbium and chromium, vanadium and hafnium by electrolysis within a fused salt bath, the procedure of providing a fused salt bath consisting of a metal oxide of the metal of the desired boride and of additional bath ingredients within which said metal oxide will dissolve at elevated temperatures in the range between about 900 and 1300 C. at which the ingredients of said bath remain molten, with said additional bath ingredients comprising a boron oxide and a substance selected from the group consisting of alkali metal oxides, alkaline earth metal oxides, alkali metal salts and alkmine earth metal salts and mixtures thereof; maintaining Within said bath a substantially continuous vessel-like porous barrier consisting predominantly of said metal oxide, passing electric heating current through the portion of said bath confined in said vessel-like barrier and therebyheating the contents of said bath to an elevated temperature within said range,V

at which said contents remain in molten condition; passing direct current betweenrran anode immersed in one region of said bath separated by said barrier from a remaining region of said bath and a cathode in contact with said remaining region of said bath for effecting therein an electrolysis and forming said metal boride deposited in the `region of said cathode whereby, during the electrolysis,

the solvent action of said molten bath on said porous barrier automatically maintains the eife'ctive concentration of the metal oxide Within the molten bath down to an amount sufciently low as to maintain said metal oxide substantially fully dissolved in the Vrmolten bath andthe formation of dispersed undissolved particles of said metal oxide within said molten bath is substantially suppressed.

3. In producing metal borides by the procedure claimed by claim 1, passing directcurrent between an anodeV immersed in a region of said bath surrounded by said `barrier and a cathode in contact with a region of said bath outside the region surrounded by said barrier.

4. ln producing metal borides by Vthe procedure claimed by claim 2, passing direct Acurrent between an anode immersed in a region of said bath surrounded by said barrier and a cathode in contact with a region of said bath outside the region surrounded by said barrier.

References Cited in the tile of this patent UNITED STATES yPATENTS 732,616 Burgess et al June 30, 1903 771,646 Von Kugelen et al Oct. 4, 1904 l,060,839 Haussr- May 6, 1913 1,196,699 Keyes et al Aug. 29, 1916 1,818,173 Suchy et al Aug. 11, 1931 1,833,425 Jessup Nov.r24, 1931 K Y FOREIGN PATENTS Y 627,947 Germany Mar.27,1936 

1. IN PRODUCING METAL BORIDE OF A METAL SELECTED FROM THE GROUP CONSISTING OF ZIRCONIUM, TITANIUM, TANTALUM, COLUMBIUM, CHROMIUM, VANADIUM AND HAFNIUM BY ELECTROLYSIS WITHIN A FUSED SALT BATH, THE PROCEDURE OF PROVIDING A FUSED SALT BATH CONSISTING OF A METAL COMPOUND OF THE METAL OF THE DESIRED BORIDE AND OF ADDITIONAL BATH INGREDIENTS WITHIN WHICH SAID METAL COMPOUND WILL DISSOLVE AT ELEVATED TEMPERATURES IN THE RANGE BETWEEN ABOUT 900* TO 1300* C. AT WHICH THE INGREDIENTS OF SAID BATH REMAIN MOLTEN, WITH SAID ADDITIONAL BATH INGREDIENTS COMPRISING A BORON OXIDE AND A SUBSTANCE SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL OXIDES, ALKALINE EARTH METAL OXIDES, ALKALI METALS SALTS AND ALKALINE EARTH METAL SALTS AND MIXTURES THEREOF; MAINTAINING WITHIN SAID BATH A SUBSTANTIALLY CONTINUOUS VESSEL-LIKE POROUS BARRIER CONSISTING PREDOMINANTLY OF SAID METAL COMPOUND, HEATING THE CONTENTS OF SAID BATH TO AN ELEVATED TEMPERATURE WITHIN SAID RANGE, AT WHICH SAID CONTENTS REMAIN IN MOLTEN CONDITION; PASSING DIRECT CURRENT BETWEEN AN ANODE IMMERSED IN ONE REGION OF SAID BATH SEPARATED BY SAID BARRIER FROM A REMAINING REGION OF SAID BATH AND A CATHODE IN CONTACT WITH SAID REMAINING REGION OF SAID BATH FOR EFFECTING THEREIN AN ELECTROLYSIS AND FORMING SAID METAL BORIDE DEPOSITED IN THE REGION OF SAID CATHODE WHEREBY, DURING THE ELECTROLYSIS, THE SOLVENT ACTION OF SAID MOLTEN BATH ON SAID POROUS BARRIER AUTOMATICALLY MAINTAINS THE EFFECTIVE CONCENTRATION OF THE METAL COMPOUND WITHIN THE MOLTEN BATH DOWN TO AN AMOUNT SUFFICIENTLY LOW AS TO MAINTAIN SAID METAL COMPOUND SUBSTANTIALLY FULLY DISSOLVED IN THE MOLTEN BATH AND THE FORMATION OF DISPERSED UNDISSOLVED PARTICLES OF SAID METAL COMPOUND WITHIN SAID MOLTEN BATH IS SUBSTANTIALLY SUPPRESSED. 